CODEWITHSUNDEEP
matlabfilter
B. Z.
B. Z.

Reputation: 129

Normalized freq response using freqz

I have the coefficients of an FIR filter,b0-b31, and I am trying to plot the magnitude and phase response of the filter.

coeff = [-8 -2 9 16 20 15 3 -19 -36 -41 -26 11 69 126 187 208 208 187 126 65 11 -26 -41 -36 -19 0 15 20 16 10 -2 -8];

when I plot it using freqz, it gives me magnitude that's not normalized as shown below: enter image description here

I am using:

freqz(coeff, 1,[],5000000) Is there a way to get the normalized plot of the magnitude and phase in the y-axis? Meaning, the maximum of the magnitude is 0 and the graph starts from it? Like the graph shown below: enter image description here

Upvotes: 1

Views: 2598

Answers (2)

Irreducible
Irreducible

Reputation: 899

There are many possibilities:

A) One way is to tell fvtool to normailize the output

fvt = fvtool(....)
fvt.NormalizeMagnitudeto1 = 'on';

B) The other option is to normalize your coefficients in the following way:

normalize_coeff=coeff/sum(coeff);

calling now 

freqz(normalize_coeff, 1,[],5000000)

will result in your desired frequency magnitude response.

As mentioned before to get both in one plot you can use fvtool.

Or you first calcultate the frequency response using the command freqz 

fs=5000000;
[H,F]=freqz(normalize_coeff, 1,[],fs);

and the phase response by using:

[phi,w] = phasez(normalize_coeff,1,fs);

and you plot both results into one graph

figure(1001)
plot(w,phi,'r')
hold on
plot(pi*(F/(fs/2)),20*log10(abs(H)))
hold off
legend('phase','magnitude')

For different y-axis you can also use the command plotyy.

Upvotes: 2

jodag
jodag

Reputation: 22214

One approach is to compute the DC gain using the output of freqz then plot the normalized filter using fvtool.

Example:

coeff = [-8 -2 7 16 20 15 0 -19 -36 -41 -26 11 65 126 178 208 208 178 126 65 11 -26 -41 -36 -19 0 15 20 16 7 -2 -8];

[h,w] = freqz(coeff);
fvtool(coeff/h(1),'freq');

enter image description here

Upvotes: 1

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